The present invention relates to a new and improved method of, and circuit arrangement for, testing, i.e. controlling the function of an ultrasonic alarm installation.
Generally speaking, the method of the present invention includes the steps of emitting ultrasonic waves of a predetermined frequency from an ultrasonic transmitter into region or space to be monitored. An ultrasonic receiver receives ultrasonic waves from the monitored region or space and converts the received ultrasonic waves into an electrical signal which is supplied to an electrical evaluation circuit. This evaluation circuit triggers an alarm signal upon a predetermined frequency deviation between the received ultrasonic waves and the emitted ultrasonic waves. A frequency deviation corresponding to an alarm condition is also generated during a test phase and simulates an alarm signal.
Such a method is known, for example, from Swiss Patent No. 612,026. Therein the fact is utilized that a person moving within the monitored region causes, due to the Doppler effect, a frequency shift in a part of the ultrasonic waves by an amount which corresponds to the person's speed of movement. This frequency shift is signaled by the evaluation circuit as an alarm condition. To this end, the frequency of the received ultrasonic waves is continuously compared with the transmitting frequency and an alarm signal is triggered at frequency deviations which are characteristic for a moving person, as described for example in Swiss Patent No. 556,070.
In the method described in Swiss Patent No. 612,026 a function control of the ultrasonic alarm installation is achieved due to the fact that during a test phase there is not utilized the pure transmitting frequency but a modulated oscillation as the comparison frequency of the receiver. The modulation is selected such that, after mixing with the receiver signal, a Doppler signal develops which corresponds to that of a moving person. Also the propagation time difference between the ultrasonic waves travelling through the monitored space and an electrical reference signal which is transmitted directly from the transmitter to the receiver or to the evaluation circit, can be utilized for the function control.
These known ultrasonic alarm installations in which ultrasound of a predetermined frequency is continuously emitted into the monitored region, however, have the disadvantage that a standing wave field is formed in the monitored region or space. Due to such standing wave field, the waves reflected at various points vectorially add to the emitted waves at the receiver and yield a receiver signal depending upon the amplitude and phase relationship. Depending upon the arrangement of the reflecting objects in the monitored region or space and the environmental conditions, the receiver signal may therefore vary within wide limits from zero to a maximum. Additionally, the signal continuously changes within wide limits, for example, due to environmental conditions such as temperature fluctuations, changes in the air humidity, or the air pressure. The sensitivity fluctuates correspondingly and uncontrollably during the test phase. In the presence of a weak receiver signal the multiplication with a modulated reference signal, however, also produces only a weak Doppler signal so that the function control becomes ineffective. Therefore even changes in the emitted ultrasonic power, e.g. due to covering of the transmitter during a sabotage attempt, cannot be distinguished from such accidental fluctuations of the receiver signal.